What is an illusion? It involves some distortion of the senses, some distortion of reality. How do you know it's an illusion? Because it would be impossible to actually have the illusion occur in real life. It describes a misinterpretation of a something decoded by the senses (as opposed to a hallucination, which involves a distortion in the absence of this type of stimulus). There are many different kinds, visual, auditory, kinesthetic, cognitive, physical, etc. Mimes, for example, are often gifted at creating illusions by physical movements such as coming up against a wall, climbing stairs, pulling or pushing and so on. I watched a mime at Pier 39 in San Francisco not long ago. It was very clear to me that there was no “wall” on the stage, and yet the mime’s physical actions were so well-matched to what one might see a person do when confronted by a real “wall” that my brain had no difficulty imagining what was happening.